1. Field of the Invention
The present invention relates to the switching control of power transistors at high frequencies, for example in the 10 to 100 KHz range.
2. Description of the Prior Art
If we consider a switching order signal S (FIG. 1) which passes from a first state or "off" state, in which the power transistor is disabled, to a second state or "on" state in which the power transistor is enabled, and rapid switching from one state to the other is desired, it is known that the base current Ib of the transistor should have the form shown in FIG. 2. As can be seen in FIG. 2, the base current Ib must be of a first polarity when the transistor is to be enabled. By convention the current which enables the transistor will be called hereafter positive base current; this convention corresponds to a current entering the base for an NPN power transistor. To cause the signal S to pass from the on state to the off state, it is not sufficient for the base current Ib to be at zero, but rather a reverse polarity current (which will be called negative base current by convention) must be initially extracted from the base. This negative current is absolutely necessary if rapid switching of the transistor from the enabled state to the disabled state is desired. In fact, only such a negative base current can eliminate fairly rapidly the electric charges which have accumulated in the base of the transistor during the conduction period. The negative base current is substantially constant during a time ts following the disabling order (destorage time), and then it comes back to zero during a time tf (fall time).
A priori, in order to give the base current this general shape or trend, two supply sources of opposite polarities must be available. This is an obvious disadvantage and so efforts have been made to construct circuits capable of supplying the negative base current without a negative supply source. For that purpose a capacitor or inductance is generally used which stores the energy for the enabled period of the transistor and restores it in the form of a negative base current at the moment when the order for disabling the transistor is initiated.
However, most of the circuits constructed up to now suffer from an important drawback: in order for the circuit to operate suitably, the capacitor or inductance must have the time to store sufficient energy during the enabled time. This means that the enabled time must have a minimum value which is not too small.
If the switching period is low (high frequency operation), this minimum value becomes a very large fraction of the switching period and the circuit can no longer be operated with a low cyclic conduction ratio, that is to say with a low ratio between the conduction time and the switching time.